This application is a national phase of PCT/NL2014/050691, filed Oct. 6, 2014, and claims priority to EP 13187458.8, filed Oct. 4, 2013, the entire contents of both of which are hereby incorporated by reference.
The invention relates to ophthalmic lenses for correcting astigmatism to be worn in or on the eye.
A typical healthy human eye has a spherical curvature over all meridians (lines of intersection with planes in which the optical axis extends) of the cornea. A common defect in the human eye is astigmatism, which is usually caused by a difference in curvature of the cornea over meridians in planes oriented at different angles. Usually the largest difference is found between meridians at an angle of 90° relative to each other. In young eyes typically most curvature is found in the vertical meridian, in older eyes typically most curvature is found in the horizontal meridian—in both cases orientations refer to eyes in the head in a normal upright position, e.g. of a standing or seated person.
Approximately 22% of cataract patients have a corneal astigmatism of 1.5 D or more. Approximately 2% have a corneal astigmatism of 4.0 D or more (Zaldanha et al. Postoperative rotation of a 3-piece loop-haptic acrylic intraocular lens. J Cataract and Refr Surg 2009;35:1751-5). Such astigmatism can be corrected by refraction inverse to the astigmatism through intraocular lenses having a toric shape. In phakic eyes (in which the natural lens is present), astigmatism can also be corrected by intraocular lenses, such as applicant's Artiflex Toric lenses, toric contact lenses or spectacle lenses.
From European patent application 0 742 461, a toric lens is known in which a toric front (anterior) or back (posterior) surface of the lens is constructed with aspheric radii in which an equation including apical vertex curvature (inverse of radius at apex) and conic constant (asphericity) parameters describes each of the prime meridians. Thus, a toric surface is obtained of which the main meridians have different prescriptions and are aspheric for providing a depth-of-focus that allows cylindrical misalignment of up to 20 degrees from the reference 90 degree or 180 degree positions so that a smaller number of lens variants for different cylinder orientations is sufficient for accommodating to variations in cylinder orientation of a patient population. However, a trade-off of depth-of-focus is reduced contrast.
Also in US 2006/0116763, aspheric intraocular lenses are described, which are shaped such that, when implanted in an eye, lens and cornea, in combination, exhibit a peak calculated modulation transfer function (MTF) contrast of at least about 0.25 and a depth of field of at least about 0.75 diopters for pupil diameters in a range of about 4.5 mm to about 5 mm for monochromatic light at a wavelength of about 550 nm. In one embodiment, the intraocular lens has one or two toric refractive surfaces that exhibit two different optical powers along two orthogonal surface directions, each with a selected degree of asphericity such that a combination of the lens and the eye in which the lens is implanted provides not only a useful image contrast, but also a depth of field as described for other (non-toric) embodiments. The depth of field of an eye in which such a lens is implanted is much higher than the depth of field of the natural eye as for instance reported in “The depth-of-field of the human eye from objective and subjective measurements”; Susana Marcos, Esther Moreno, Rafael Navarro; Vision Research 39 (1999) 2039-2049.
From European patent application 1 903 986, an intraocular lens with a toric surface on its front surface as well as on its back surface is known. Also in this lens, the shape of the main meridians is described by an equation including the radii at apex and asphericity parameters. The radius at apex and asperities of intermediate meridians between the main meridians is determined as a sinusoidal function of the respective values of the main meridians, dependent on the angle of the intermediate meridian relative to the main meridians. Manufacturing a lens with toric surfaces on both sides increases manufacturing complexity and costs.
Current toric intraocular lenses and contact lenses comprising aspheric toric surfaces introduce additional aberrations into the human pseudo phakic optical system. In particular, such aberrations increase with the pupillary opening when a larger lens surface is involved in refracting light onto the retina. Since the pupillary opening is generally larger in low light circumstances, this entails that contrast deteriorates the more it is needed for adequate vision in view of unfavourable lighting conditions, so that night time visual acuity and light perception is adversely affected.